This invention relates to optical transmitting devices. More particularly, this invention relates to methods and apparatus for analyzing polarization mode dispersion in optical transmitting devices, such as single mode optical fibers.
Long distance transmission of digitally modulated information through an optical fiber is hampered at high bit rates by pulse distortion caused by chromatic dispersion and polarization mode dispersion (hereinafter, "PMD"). In addition to fiber birefringence, PMD is caused by intrinsic and extrinsic perturbations in the optical fiber. Intrinsic perturbations P.sub.i are local defects that exist in a fiber that cause random misalignment .theta. between the Stokes vectors of the Principal States of Polarization (hereinafter, "PSP") of light and the local axes of birefringence. The average spacing L.sub.i between intrinsic perturbation P.sub.i is given by the length L of the fiber divided by the total number N.sub.i of intrinsic perturbations that occur in that length. In contrast to intrinsic perturbations, extrinsic perturbations Pe correspond to disturbances that are produced at the physical splices between optical elements that make up a composite fiber.
PMD is especially limiting at high digital transmission rates (e.g., at rates of gigabytes per second or more). In particular, PMD causes light to split into two PSP with two group delays. See, e.g., Poole et al., "Phenomenological Approach to Polarization Dispersion in Long Single Mode Fiber," Electronics Letters, Vol. 22, September 1986, pp. 1029-1030 (hereinafter, "Poole et al."). Furthermore, during transmission through optical fibers PMD can lead to broadening of optical pulses. Accordingly, PMD may limit transmission to low frequencies and relatively short optical fiber lengths.
General and rigorous mathematical models have been developed to describe these PMD limitations, including the models developed by Poole et al. and Foschini et al. "Statistical Theory of Polarization Dispersion in Single Mode Fiber," Journal of Lightwave Technology, Vol. 9, November 1991, pp. 1439-1456. These and other models detail the PMD dependence on time, temperature, and wavelength in an optical fiber. Generally, such models involve theoretically "constructing" an optical fiber from an arbitrary number of concatenated optical elements and allowing certain optical parameters, such as the birefringence, to vary randomly according to an arbitrary distribution. Therefore, if the PMD dependence on wavelength of a real optical fiber is known (i.e., measured), a theoretical fiber can be modeled, or constructed, by varying the optical parameters of the optical elements that make up that fiber until the theoretical fiber has a PMD dependence on wavelength that is similar to that of the real fiber. These prior theoretical models, however, do not show or suggest any way of analyzing individual optical elements that make up the fiber using measured PMD data for the entire fiber.
It is therefore an object of the present invention to provide a method for analyzing individual optical elements that make up an optical device.
It is also an object of the present invention to provide a method for determining how PMD, or any other optical and physical parameter that characterizes the device, varies statistically along the length of the device.
It is a further object of this invention to provide apparatus for use with these methods.